This invention is related to a solid polycrystalline diamond cutting tool, and a method for generating a prescription surface in a plastic lens used in spectacles, and more particularly to a method for generating a surface in a plastic lens blank that eliminates the finishing step required after the conventional lens grinding step.
Plastic lens blanks are conventionally formed in an injection molding process, either of a polycarbonate or a plastic material commonly known as CR-39. Polycarbonate is much tougher than CR-39 and is used where safety is a factor in material selection.
U.S. Pat. No. 3,685,210 which issued Aug. 22, 1972 to Gilbert John Bowen shows a prior art method for generating a surface on a lens.
Most commercial processes employ grinding tools for generating the lens. The steps in the conventional grinding process is as follows:
1. A prescription is delivered to the lens generating lab from the optometrist. The prescription is used to set the generating machine. PA1 2. The front side of a lens blank is coated with a dark blue blocking liquid. When dry, the blocking liquid functions to form a temporary bond with a support block. The drying time for the liquid constitutes the longest step in the generating process. PA1 3. The lens blank is then mounted in a blocking machine. A metal support block is placed on the coated front surface and provides means for supporting the lens during the generating, finishing and polishing steps. PA1 4. The blank with the block is then mounted on the generating machine. The generating machine is a standard machine which has been in use for use for many years. Typically a grinding tool is employed for forming the backside of the lens. The tool is rotated at a high speed while traversing the back side of the lens in a series of passes. Each pass removes a layer of material. However, the amount of material removed during each pass is limited by the nature of the grinding surface. Typically, a grinding tool will remove about 2 millimeters per pass, but not limited to. Since as much as 12 millimeters of material has to be removed, several passes are required. The product of the generating machine is a surface having the curvature of the prescription but which is not a finished surface. There is a considerable amount of wastage because of the nature of the grinding process. PA1 5. The lens then is transferred to the first of two cylinder finishing machines. In the first machine, the lens is finished with a pair of abrasive pads in a vibratory process. Each pad is mounted on a metal lap and vibrated against the lens, with water employed as a coolant for 45 seconds to a minute and half. A second abrasive pad is then used, also employing water. PA1 6. The pads are mounted on the lap by an adhesive substance. When the lens has been finished, the adhesive pad which forms a tight attachment, must then be removed. This is a major time-consuming process. In some large shops, some personnel are assigned primarily for peeling the abrasive pads from the lap. PA1 7. The second finishing machine also employs a metal lap on which a thin felt-like pad is used for polishing the lens. The polishing step uses a slurry with the pad. The final polishing step takes about one to five minutes.
The entire process takes about 18 to 20 minutes and in many cases requires a considerable amount of reworking. The felt-like finishing pad is also mounted by an adhesive on a metal lap and has to be peeled after each lens.
In some cases a rotating cutting tool has been employed, rather than a grinding tool. The cutting tool uses an insert having a generally flat surface with a semi-circular cutting edge, however, only a small portion of the edge is formed of a solid polycrystalline diamond material. The problem is the tool performance deteriorates as the tip becomes chipped or worn. Further, it is difficult to cut some curvatures.
Further, the tool is usually swung in a pivoting motion during each pass. A cutting element having a cutting edge only partially formed of a solid polycrystalline material will only cut the lens material when swung in one direction. It will not cut in a reverse motion which limits the usefulness of the generating machine.